1. Field of the invention
The present invention relates to a method and an apparatus for the gain stabilization and the calibration of a signal issued from a nuclear detector device.
2. The prior art
Such devices are commonly used in many technical areas where measurements involve nuclear particles and radiation detection, one among others being e.g. the well logging techniques, wherein a tool is lowered in a well to carry out physical measurements.
Of the many well logging instruments and techniques developed over the years to determine the characteristics, such as the hydrocarbon content and productivity, of earth formations, the nuclear spectroscopy tool, by which energy spectra of the constituents of formation matrices and fluids are generated, more specifically in which gamma rays are detected, has been found to provide information of particular value in formation analysis.
These gamma rays can be due to the natural radioactivity of the formations or result from the irradiation of these formations by a neutron or gamma ray source. The detection of these gamma rays, whether from natural or induced radioactivity, can be carried out separately for several distinct windows or energy ranges in order to obtain information on the energy spectrum of these gamma rays. Analysis of the obtained spectrum then furnishes information on the lithology of the formations.
As an example, an apparatus for analyzing the spectrum of natural gamma rays is described in U.S. Pat. No. 3,976,878 (P. Chevalier and B. Seeman). In that apparatus, the detection system comprises a scintillation crystal, a photomultiplier, a linear amplifier and a pulse height analyzer. In the pulse height analyzer, the energy spectrum of the gamma rays is divided into five windows. The count rates of the different windows make it possible to calculate the thorium, uranium, and potassium (T,U,K,) contents of the formations. In this technique, as in all those using a spectral study i.e. discrimination by the energy of the received radiation, it is extremely important for measurement accuracy to equip the detection system with gain stabilization means. In fact, gamma ray detection systems, and in particular photomultipliers and crystals, exhibit significant gain variations due especially to temperature changes or changes in the count rates. A stabilization method frequently used, and described in the above mentioned patent, consists of an auxiliary gamma ray source that emits a characteristic gamma ray whose energy is below the range of interest in the detected spectrum. In this method, an Americium source (Am 241) is chosen, the primary emission peak of which is located at 60 KeV. Two count rates M.sub.1 and N.sub.1 are measured in two windows of predetermined width located on each side of the 60 KeV energy and an error signal, which is a function of the difference (M.sub.1 -N.sub.1), is used to control the high voltage of the photomultiplier through a negative feedback loop. The above described stabilization technique is suitable for those portions of the spectrum close to the emission peak. However, for the same gain variation, the spectrum energy scale is shifted by greater amounts at higher energies. Therefore, the detection of the shift at low energies is not entirely satisfactory for correcting shifts at higher energies and, due to statistical variations in the count rates, errors appear. In addition, when large scintillator crystals are used, the low energy gamma rays of the auxiliary source reach only a part of the crystal, which is all the more smaller when the crystal has high efficiency. Thus, the resulting stabilization is seriously affected by any crystal heterogeneity and/or temperature gradients in the crystal.
The U.S. Pat. No. 3,101,409 (L. E. Fite) proposes a stabilization using two peaks coming from two auxiliary gamma ray sources. One of the peaks is used to control the high voltage of the photomultiplier while the other peak is used to control the lower threshold of the pulse height discriminator. The two stabilization loops are independent and do not make it possible to solve the case in which one of the peaks in not present. In this patent, this problem is not addressed because the peaks come from two auxiliary monoenergetic gamma ray sources especially added to the apparatus for stabilization.
According to U.S. Pat. No. 3,922,541, a gain stabilization method is proposed wherein a reference radiation source (having a predetermined energy spectrum) is located adjacent to the scintillator. A reference signal representative of the relationship between the predetermined energy spectrum from the reference radiation source is produced.
It has also been proposed, in U.S. Pat. No. 4,433,240 (B. Seeman), a gamma ray detection apparatus that includes a stabilization loop based upon the detection of several peaks at different energy levels located in the energy range of the detected spectrum. Discrimination means separates the electric pulses, representative of the gamma rays, the amplitudes of which fall within two first contiguous windows located on each side of a first predetermined value and within two second contiguous windows located on each side of a second predetermined value. These predetermined values correspond to two reference energies or peaks located in the detected spectrum of gamma rays coming from the formations. This known apparatus further includes another stabilization loop based upon an auxiliary gamma ray source the emission peak of which is located outside of the detected spectrum of the formations. This apparatus, although being an improvement over the above mentioned apparatus, relies on high energy peaks which are part of the measurements themselves. Since these energy peaks have to be removed from the total energy spectrum, this decreases the reliability of the measurements.
Another gain stabilization method has been proposed, by using a reference signal, either of the electrical type or of the light type, such as that of U.S. Pat. No. 4,220,851 including a light diode driven by a pulser, mounted between the scintillator and the photomultiplier, and emitting light pulses constituting reference pulses of constant energy (above 8 Mev). However, the resulting gain stabilization is based on a reference signal which does not affect the crystal, thus does not obviate any drift occurring in the latter. Furthermore, the high reference energy (above 8 Mev) used does not fit the energy range encountered in current spectra analysis (usually below 1.5 or 2 Mev). Finally, this method relies on the yield stability over the time of the light source.
In the same vein, U.S. Pat. No. 3,900,731 contemplates a method and apparatus for stabilizing the gain of a photomultiplier by compensating its variations through a modification of the illumination of its cathode.
Gain stabilization methods based on the measurements of coincident nuclear events have also been proposed.
In this respect, U.S. Pat. No. 2,769,916 shows a neutron detecting device provided with two facing scintillation detectors between which is disposed a foil of neutron-gamma reactive material, i.e. able to produce, upon bombardment by an incident neutron, gamma rays, two of which are emitted simultaneously and thus detected simultaneously in both detectors. However, this device does not provide any gain stabilization.
U.S. Pat. No. 4,450,354 (H. D. Smith and C. A. Robbins), depicts a method for the natural gamma ray detection of the casing thickness in a cased well borehole, using an ancillary detector which is substantially smaller than the regular detector used for the measurements. The gain stabilization is achieved by using an ancillary nuclear source (Am 241) emitting practically simultaneously an alpha particle which is detected by the ancillary small detector, and a photon-gamma radiation of 60 Kev which is detected only by the regular large detector. A coincidence circuit, each time an alpha particle and a photon-gamma are simultaneously detected, actuates a stabilizing circuit which increases or decreases the amplifier gain depending upon the actual energy (as measured) of the photon-gamma particle being below or above the theoretical energy value of 60 Kev. However, this method is not fully satisfactory. Firstly, the reference energy peak is in the low part of the energy range. Secondly, the ancillary source and the ancillary detector (plus the shields) increases the complexity of the logging tool, thus the cost and size. Thirdly, the two detectors are different with regard to (i) size: one is small and the other is large, (ii) material: one detects alpha particle, the other detects gamma rays, and (iii) use: only one detector is sensitive to particles representative of the earth formation.
It can be seen from the above that the attempts made until now towards a better gain stabilization are not fully satisfactory.
Moreover, the safety concern with respect to nuclear sources has indubitably increased over the years. Accordingly, the regulations have become more and more stringent. For example, the activity (measured in microCurie or nanoCurie) of the sources should not exceed a given value. It is, however, difficult to determine and find a nuclear source which complies with the regulations as well as the needs of the industry.
Furthermore, there is a demand for improving the sensitivity of the detectors commonly used, such as NaI (sodium iodide) detectors. For instance, U.S. Pat. No. 3,633,030 shows a logging tool including means attempting to improve the energy resolution of the detector. It includes one or two NaI scintillator(s), disposed close to a semiconductor radiation detector which, upon receipt of a gamma rays coming from the formation, emits an electron and a positron. The positron produces two simultaneous, oppositely directed gamma rays, which are detected by each detector. Scintillator signals are applied to a gating circuit which passes or inhibits the signal from the semiconductor detector, thus allowing delivery of a coincidence or anticoincidence spectrum. This device does not provide any gain stabilization, and is directed to the elimination of the Compton production radiation. Furthermore, the semiconductor requires thermal protection, which increases the costs.
Moreover, as a general consideration, the higher the count sensitivity of a nuclear detector, the bigger its size. This leads to an increased bulk and to an enhanced difficulty in stabilizing the spectrum with a low energy peak, since only a small part of the detector is actually affected by the ancillary source, and thus is not representative of the entire detector.
Accordingly, there is a general need for a method and apparatus for stabilizing the nuclear spectrum signal generated by nuclear detectors, and thus correcting (i) for any offset, wherein the spectrum is uniformly translated, and (ii) for any gain drift which implies the spectrum is "stretched".